To form identical twins,
one fertilised egg
(ovum) splits and
develops two babies
with exactly the same
genetic information.
This differs from
fraternal twins, where
two eggs (ova) are
fertilised by two sperm
and produce two
genetically unique
children, who are no
more alike than
individual siblings born
at different times.
Twins are more or less
equally likely to be
female or male.
Contrary to popular
belief, the incidence of
twins doesn’t skip
generations.
Factors that
increase the odds
of having twins
Some women are more
likely than others to
give birth to twins. The
factors that increase
the odds include:
Advancing age of the
mother – women in
their 30s and 40s have
higher levels of the sex
hormone oestrogen
than younger women,
which means that their
ovaries are stimulated
to produce more than
one egg at a time.
Number of previous
pregnancies – the
greater the number of
pregnancies a woman
has already had, the
higher her odds of
conceiving twins.
Heredity – a woman is
more likely to conceive
fraternal twins if she is
a fraternal twin, has
already had fraternal
twins, or has siblings
who are fraternal
twins.
Race – Black African
women have the
highest incidence of
twins, while Asian
women have the
lowest.
Assisted
reproductive
techniques – many
procedures rely on
stimulating the ovaries
with fertility drugs to
produce eggs and,
often, several eggs are
released per ovulation.
